# 2nd Year Chemistry Problem?

A 2.0 g Samples of C2H2 (g) is burned in a bomb calorimeter at 25%. 23.8 kcal of heat energy are evolved. for the combustion of 1.0 mole of C2H2 (g) at 25 degrees C. Calculate the Delta E and Delta H.
I already know the answer just need to know how to set it up.

Answer:
Delta E? What does E stand for? Do you mean energy of formation?

For deltaH of combustion:

2g C2H2 = 2g/26g/mol = 0.07682 mol
DeltaH = - 23.8kcal/0.07682mol =-309.8 kcal/mol of C2H2
(negative comes out b/c rxn is exothermic)

For heat of formation:
Balanced combustion rxn:
C2H2 + (2.5)O2 ~> 2CO2 + 1H2O
deltaH = (heat of formation products) - (heat of formation reactants)
Now using the coefficient from the equation this becomes
(-309.8 kcal) = 2(CO2)+(H2O)-(C2H2). Each compound in parenthesis represents its own heat of formation.
Solve the equation for C2H2:
C2H2 = {2(CO2) +H2O + 309.8kcal}
Plug in the values with the appropriate units:
(CO2) = -393.5 kJ/mol
(H2O) = -241.8 kJ/mol
309.8kcal = 1296 kJ/mol
This gives (C2H2) = 267.5kJ/mol = 63.94 kcal/mol
2C2H2 + 5O2 -> 4CO2 + 2H2O

Now calculate.

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